1. Field of the Invention
The present invention relates to a semiconductor device having a circuit formed of a thin film transistor (hereinafter referred to as a TFT) on an insulator (substrate) having an insulating surface, and to a manufacturing method thereof. In particular, the present invention relates to an electro-optical device represented by a liquid crystal display device where a pixel portion and a driver circuit provided on the periphery thereof are provided on the same insulator, and to an electric apparatus (electronic device) having such an electro-optical device mounted thereon. It is to be noted that a semiconductor device as used herein means any device which functions by utilizing semiconductive characteristics, and the above-mentioned electro-optical device and electric apparatus having the electro-optical device mounted thereon are included in semiconductor devices.
2. Description of the Related Art
Development of semiconductor devices having a large-area integrated circuit formed of TFTs on a substrate having an insulating surface is making progress. Known typical examples of such semiconductor devices include liquid crystal display devices, EL (electroluminescence) display devices, and contact type image sensors. In particular, a TFT having a polysilicon film (polycrystalline silicon film) as an active layer (hereinafter referred to as a polysilicon TFT) has a high field-effect mobility, and thus, is used in circuits having various functions.
For example, in an active matrix liquid crystal display device, a pixel portion for displaying an image and a driver circuit based on a CMOS circuit and including a shift register, a level shifter, a buffer, a sampling circuit, and the like are formed on the same substrate. In a contact type image sensor, a driver circuit for controlling a pixel portion including a sample hold circuit, a shift register, a multiplexer, and the like is formed using TFTs.
In a pixel portion of an active matrix liquid crystal display device, a TFT (hereinafter referred to as a pixel TFT) is disposed in each of several tens to several million pixels, and each of the pixel TFTs is provided with a pixel electrode. An opposing electrode is provided on the side of an opposing substrate beyond liquid crystal, and a kind of capacitor with the liquid crystal being as a dielectric is formed. Voltage applied to the respective pixels is controlled by the switching function of the pixel TFTs to control the electric charge of the capacitors, thereby driving the liquid crystal, controlling the light transmission amount, and displaying an image are achieved.
However, since the stored capacitance of the capacitors gradually decreases due to leakage current caused by, for example, OFF current of the pixel TFTs (drain current which exists in spite of the OFF state of the TFTs), the light transmission amount varies to decrease the contrast in image display. Therefore, conventionally, capacitors (storage capacitors) for compensating for the capacitance lost by the capacitors with the liquid crystal being as a dielectric are additionally provided in parallel with the capacitors with the liquid crystal being as a dielectric.
As examples of such a storage capacitor provided in the pixel portion of an electro-optical device, the applicant of the present invention has already filed Japanese Patent Application Nos. Hei 11-045558, Hei 11-053424, and Hei 11-059455, which disclose a storage capacitor that is formed of a shielding film (or a light shielding film), an oxide formed on the shielding film, and a pixel electrode.
However, there is a fear that the storage capacitor disclosed in the above-mentioned applications may cause a problem, which is described in the following. With reference to FIG. 3, a shielding film 302 made of a metal film which blocks light is formed on an insulating film 301 made of a resin material, and an anodic oxide 303 formed by anodic oxidation and a pixel electrode 304 are formed on the shielding film 302 to form a storage capacitor. However, if there is dust or the like on the shielding film 302 when the anodic oxide 303 is formed, that portion may not be anodically oxidized to form a minute hole (hereinafter referred to as a pin hole) 305.
If the pixel electrode 304 is formed with the pin hole 305 existing, there is an inconvenience that the shielding film 302 and the pixel electrode 304 are short-circuited through the pin hole 305. In other words, a region which causes electric leakage or short circuit is formed between a pair of electrodes.
With reference to FIG. 4, a storage capacitor made of a shielding film 402, an anodic oxide 403, and a pixel electrode 404 is formed on an insulating film 401 made of a resin material. Reference numerals 405, 406, 407, and 408 denote an alignment film, an opposing substrate, an opposing electrode, and an alignment film, respectively. Liquid crystal 410 is retained between the alignment films 405 and 408 with the help of a spacer 409. The spacer 409 is provided for securing a cell gap in a liquid crystal cell.
Here, if the insulating film 401 made of a resin material is not flat enough, the top of the TFT 411 is higher than other regions. The storage capacitor is, since it is formed on the TFT 411, still higher, and thus, there is a step at the height H in the cell gap. If the spacer 409 is by chance disposed over the storage capacitor, the spacer is compressed in the process of stacking the liquid crystal cell, which may crush and break the storage capacitor. In particular, if the spacer is a bead-like spacer such as a silica ball, since such a spacer is very hard and force applied by such a spacer concentrates on a point, compression by such a spacer easily causes a crack in the pixel electrode.
Though the above-described inconvenience does not always occur, it can be a factor of decreasing the yield.